Week number
The ISO week of any date, with its boundaries.
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“See you in week 38” — but when is that?
Factory schedules, deliveries, holidays, sprints: the week number paces European business life. Pick a date (today by default): the tool shows its ISO week, the exact Monday-to-Sunday dates, the day of the year and the year’s week count.
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Pick the date
Today in one click, or any past or future date.
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Read the ISO week
With its ISO year — which can differ from the calendar year in early January and late December.
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Check the boundaries
The exact Monday and Sunday remove any scheduling ambiguity.
The ISO 8601 rule in short
- Weeks run from Monday to Sunday.
- Week 1 contains the year’s first Thursday (and therefore always January 4).
- A year has 52 weeks, or 53 if it starts on a Thursday (or Wednesday for leap years).
- The first 1-3 days of January can belong to week 52/53 of the previous year, and vice versa in late December.
Excel defaults to another convention (WEEKNUM returns the US system): use ISOWEEKNUM — or this tool — for numbers matching European schedules.
Frequently asked questions
How is the week number calculated?
Per the ISO 8601 standard, used across Europe: weeks start on Monday, and week 1 is the one containing the first Thursday of January (equivalently, the one containing January 4). A year therefore has 52 or 53 weeks.
Why can January 1 be in week 52 or 53?
If January 1 falls on Friday, Saturday or Sunday, it belongs to the last week of the previous year. Conversely, late December can already belong to next week 1. That is why the tool also shows the “ISO year”.
Which years have 53 weeks?
Years starting on a Thursday, and leap years starting on a Wednesday. That happens roughly one year in five or six — 2026 has 53, for instance.
Does the US count the same way?
No: the US convention starts weeks on Sunday and places week 1 at January 1. Numbers can differ by one — always say “ISO week” in international exchanges.